1. Field of the Invention (Technical Field)
The present invention relates to magnetometers, and specifically to pulsed free induction decay nonlinear magneto-optical rotation (NMOR) magnetometers and corresponding methods.
2. Description of Related Art
Sensitive magnetometers are important in a variety of fields, including defense applications such as submarine detection and mineral exploration. Atomic magnetometers feature high sensitivity to the magnitude of the total magnetic field, making them easier to deploy because the signal is almost independent of the orientation of the sensor with respect to the field. A number of companies manufacture atomic magnetometers, including cesium magnetometers by Geometrics (San Jose, Calif.) and Scintrex (Concord, Ontario Canada) and potassium magnetometer by GEM Systems (Markham, Ontario Canada). These commercial magnetometers are based on the original work of Bell and Bloom [W. Bell and A. Bloom, “Optically driven spin precession,” Phys. Rev. Lett. 6, 280-281 (1961).] In such magnetometers, visible or near-infared radiation is applied to an atomic vapor with the wavelength chosen to be resonant with one of the allowed electronic transitions. The optical radiation polarizes the atoms. A radiofrequency (RF) field is also applied. When the frequency of the RF field is resonant with the Larmor frequency of the atoms, the field induces transitions among the magnetic sublevels, such that the optical properties of the atomic vapor are changed. A change in transmission can be observed. These magnetometers typically achieve a sensitivity of 1 to 10 picoTesla in a 1 Hz bandwidth. In addition, the RF field has a long wavelength and thus is difficult to confine to the atomic vapor. When such magnetometers are placed in close proximity it is possible for the RF field from one device to influence the performance of the neighboring device, possibly degrading the accuracy with which the field is measured.
In addition to observing spectroscopic transitions among the magnetic sublevels in the frequency domain, it is possible to observe the transitions in the time domain. When a sample is excited with a pulse, a damped oscillation decay signal known as free induction decay (FID) can result. The oscillation frequency has information equivalent to the transition frequencies of the spectrum observed the frequency domain, and the damping envelope has information equivalent to the width of the transition in the frequency domain. FID is known in nuclear magnetic resonance (NMR) and optical spectroscopy. See, e.g., W. Demtroder, Laser Spectroscopy, Basic Concepts and Instrumentation, 3rd ed, Springer Verlag, New York (1988), pp. 580 ff.
Nonlinear magneto-optical rotation (NMOR) is a recently developed technique for measuring magnetic fields with high sensitivity. A typical NMOR apparatus includes a low pressure cell that contains atomic vapor and that has at least one window for admitting light to atoms and for transferring light to a detector. The apparatus also includes a linearly polarized light source that can be tuned to a spectral feature of the atomic vapor. Polarizers for analyzing the light transmitted through the atomic vapor, and photo-detectors for converting light intensity to an electrical signal are also included. The cell is a glass bulb or a glass tube, the inner surfaces of which are treated with a hydrocarbon to suppress wall relaxations. The light source is a diode laser. The photodetector is a photodiode. Fiber optics deliver the light beam and receive the return beam.
FIG. 1 illustrates a schematic diagram of a known amplitude-modulated (AM)-NMOR apparatus. In this diagram, a diode laser is used as a light source. The wavelength of the laser is stabilized at a target point on the spectral feature. A dichroic atomic vapor line locking apparatus (DAVLL) is used for stabilization. Separate pump and probe beams are used, and the amplitude of the pump beam is modulated with a Mach-Zender modulator (MZM). A magnetic-field probe comprises an atomic vapor sample in an anti-relaxation-coated glass cell, and also comprises polarization optics for defining the polarization of the probe beam and analyzing it. The detection scheme can be differential detection as illustrated in FIG. 1, or a single detector can be used. Better sensitivity is usually obtained when the analyzing polarizer is set so that equal optical power falls on two detectors, so that their difference measures the time-dependent polarization rotation while cancelling laser amplitude noise. When a single detector is used, the analyzing polarizer is set at a point that balances detector noise and source noise.
When a light source is tuned to an atomic spectral feature, the light source pumps the atoms in the cell, causing an alignment of the atoms as a result of coherence between the magnetic sublevels of the ground state. In one NMOR technique, a laser is operated continuously, which is capable of measuring fields only when they are very close to zero. The Larmor precession frequency is less than the relaxation rate. For typical relaxation rates of about 10 s−1 and tuning rates of about 10 Hz/nT, the measurement range is about 1 nT.
Both frequency-modulated NMOR (FM-NMOR) and amplitude modulated NMOR (AM-NMOR) are available modulation techniques. These modulation techniques can be used to measure higher magnetic fields, at least up to Earth's field and well beyond, by using stroboscopic pumping of the atoms. However, in FM-NMOR and AM-NMOR the modulation frequency is carefully matched to the Larmor precession frequency of the atomic sample or one of its harmonics, typically within about 1/relaxation rate. S. Pustelny et al., “Nonlinear magneto-optical rotation with modulated light in tilted magnetic fields”, Phys. Rev. A 74, 063420 (2006). Such matching is accomplished by tuning the modulation frequency or by operating the magnetometer in a self-oscillating configuration. However, the need to precisely match the modulation frequency is a problem when measuring the magnetic field at several points, as in a gradiometer or an array detector, because typical spatial variations in the magnetic field over distances of one meter result in Larmor frequencies that differ by more than a typical line width of 1 nT. WO 2009/073256 describes the measurement of magnetic fields by NMOR using modulation of the frequency or amplitude of a light source. However, the modulation itself is continuous. The modulation frequency must be closely matched (typically, to within less than 10 Hz) to a harmonic of the Larmor frequency. A separate modulator is needed for each measurement point. This can add significant cost and complexity to the system. In addition, technical problems can arise. When two channels are used to measure a gradient by self-oscillating AM NMOR, inadvertent locking of the two channels can occur as a result of coherent electrical noise.
U.S. Pat. No. 7,573,264 and WO 2009/079054 describe a method and apparatus for detecting RF fields, including the free induction decay magnetic field generated by a sample. The invention applies RF pulses to the nuclei of a sample under study. However, the magnetometer itself operates with continuous modulation, which must be matched to a harmonic of the Larmor frequency. This prior art does not disclose means for creating a single optical pulse or a short burst of optical pulses to excite the atoms in the magnetometer. As a result, if it is desired to measure fields at a number of locations simultaneously, then each measurement point would need to be provided with its own modulation means.
Additional arguably related references are next discussed. WO 2009/073740 and WO 2009/073736 disclose a technique for manipulating spins in a solid state lattice. Like a Bell-Bloom magnetometer, both an optical field and an RF field are required. RF pulses control the spins, while a laser reads out the spin state. Separate RF pulses are polarized along x and y.
More recently, all-optical magnetometers have been introduced in the laboratory. These include spin-exchange-relaxation free (SERF) magnetometers (U.S. Pat. No. 7,038,450). SERF magnetometers achieve high sensitivity but require high sample temperature to achieve a high density of atoms. SERF magnetometers also operate at relatively low magnetic fields (less than 0.1 microTesla), so that operation at fields typical of Earth's surface (20-80 microTelsa) requires the use of a stable magnetic field which can reduce Earth's field to a level where the magnetometer can operate.
Pulsed excitation has also been used to improve the performance of a fluxgate magnetometer. U.S. Pat. No. 7,378,843. The fluxgate magnetometer is a different measurement approach from atomic magnetometers. The fluxgate includes a magnetic core and at least one plurality of windings. Pulses of electric current of either a positive or alternating sign are applied to one of the windings. The pulse frequency can be chosen arbitrarily to optimize the bandwidth. The FID decay of the resulting signal cannot be used to measure the magnetic field by analyzing its frequency content. The calibration of the fluxgate magnetometer depends on the calibration of the windings and of the current pulse.
The present invention is an optical magnetometer that does not require RF fields, so that two magnetometers can be placed in close proximity without cross-talk between the readings. The magnetometer does not require reducing the magnetic field below the value typical of Earth's surface. The burst period need not closely match the Larmor frequency, so it is possible to measure magnetic fields at a number of points using a single modulation means. The measurement of the magnetic field depends on the properties of the atomic vapor, resulting in stable calibration.
There is currently a need for an NMOR that does not have the inherent problems of the NMOR techniques discussed above. Embodiments of the present invention solve these problems.